Hydropneumatic suspension unit with automatic level regulation

ABSTRACT

A self-pumping hydropneumatic suspension unit is provided with automatic level regulation operative even when the vehicle is stationary. The suspension-unit housing holds an electric pumpdriving motor and also a regulating valve. A level-control switch electrically controls operation of the motor on the one hand and of the valve on the other hand in dependence on the vehicle load.

ild t n ntnl [mentor Erich lllahn Ennepetal, Germany App]. No 82%,297 Filed Mny 27,1969 Patented Sept. 7, 1971 Assignee Eirrna August lBillstein Ennepetal, Germany Priority June 10, 11968 Germany P 117 55 693.3

HYDIROIPNEUMATIC SUSPENSKON lUNll'lf Wll'l'lll AUTOMATIC LEVEL REGULATION 19 Claims, 7 Drawing lFigs.

LLS. Cl 267/64 Int. Cl lFllSf 5/00 Field oi lurch 267/64, 64 A, 64 B, 65

[56} llelerenm Cited UNITED STATES PATENTS 3,181.877 5/1965 McHenry 267/64 3,222,048 12/1965 Wilkins 267/64 Primary Examiner1ames B. Marbert Auorney-liarl IFv Ross ABSTRACT: A self-pumping hydropneumatic suspension unit is provided with automatic level regulation operative even when the vehicle is stationary. The suspension-unit housing holds an electric pump-driving motor and also a regulating valve. A level-control switch electrically controls operation of the motor on the one hand and of the valve on the other hand in dependence on the vehicle load.

PATENTED SEP 7 I971 sum 2 or 4 DEL REL Fig. 4

In ven for:

ERlCH HAHB 5. (Raf 1h ATTORNEY PATENTED SEP 7197i SHEET 3 OF 4 Fig. 6

BRUSH HAHN In ventor's ATTORNEY PATENTEUSEP 7|97l SHEET t [1F 4 ERLCH HAHN ATTORNEY HYDROPNEUMATIC SUSPENSION UNIT WITH AUTOMATIC LEVEL REGULATION This invention relates to hydropneumatic suspension units with automatic level regulation, particularly for use on motor vehicles.

The invention relates more particularly to hydropneumatic suspension units with automatic level regulation which comprise a working cylinder filled with working liquid, a working piston arranged to be slidable in the working cylinder, a piston rod secured to the working piston and having one end sealingly projecting from the working cylinder, a pressurized gas chamber maintaining the working liquid in the working cylinder under pressure, a reservoir chamber partially filled with working liquid, a pump located between the reservoir chamber and the working cylinder, and a regulating valve for the working liquid.

In certain known hydropneumatic suspension units of this type, generally referred to as self-pumping suspension units, the relative movements which occur during movement of the vehicle between the working cylinder and the piston rod are used to pump a part of the working liquid from the reservoir chamber into the working cylinder for continuous level regulation, in order that by this means a corresponding rise in pressure occurs in the cylinder which leads to an increase in the extension force exerted on the piston rod.

Various types of pump are known for this purpose, and in these pumps the pumping chamber is generally located within the hollow piston rod while a plunger is secured to the base of the working cylinder and extends to a greater or lesser degree into that piston rod. As soon as the desired level position has been achieved by suitable forced circulation of the working liquid, the pumping action is automatically discontinued. If the piston rod extends too far from the working cylinder, for example after unloading of the vehicle, the working pressure is then also too high, the regulating valve opens, and some of the working liquid flows back into the reservoir chamber and consequently causes a corresponding pressure drop in the working cylinder. All these known self-pumping hydropneumatic suspension units have the important disadvantage that the automatic level regulation can be achieved only after sufficient vehicle movement. No level regulation can take place with a stationary vehicle. This is a serious drawback since at the start of a journey there may be considerable danger due to the unbalanced suspension state of the vehicle. Moreover, it has also been observed that even with vehicles that have been stationary for only a few hours the self-pumping suspension units lose piston-rod-extension power on account of unavoidable seepage and leakage losses at the liquid valves, particularly the regulating valve, as a result of which the suspension units must as a general rule be first pumped up when the vehicle begins to move.

In another known type of resilient vehicle-suspension system, in which several suspension units connected into a compound hydropneumatic assembly can be maintained by a pump and control unit mounted on the side of the vehicle and can thereby be brought to or maintained at a given level, level regulation can also be effected when the vehicle is stationary. However, such compound suspension-unit systems require extensive hydraulic or compressed-air pipework between the vehicle and the individual suspension units. These supply pipes not only create considerable problems in mounting them on the vehicle but also require very careful maintenance and supervision. If there are defects in the piping or if any damage occurs to one of the suspension units, the entire pipe network must be disconnected or sealed off in separate sections so that the pressurized medium can be drained off and the system then refilled after repair. All this requires very considerable repair work which can be undertaken only in a workshop.

It is an object of the present invention to provide a hydropneumatic suspension unit with automatic-level regulation which, as in the self-pumping types of unit described above, can be used as an independent suspension unit, but which can also be used to obtain the desired-level regulation even when the vehicle is stationary.

A self-regulated hydropneumatic suspension unit according to my invention comprises, in addition to the usual upright piston-and-cylinder combination in. a casing which forms a reservoir for the working liquid, the following elements:

a. a normally inoperative pump in a first passage between the liquid reservoir and the working chamber defined by the piston inside the the cylinder;

b. a normally closed regulating valve in a second passage extending between the reservoir and the working chamber;

c. a drive motor supported by the casing for operating the pump to feed liquid from the reservoir to the working chamber;

d. an electric control means, such as an electromagnet, for opening the regulating valve to enable a return flow of liquid from the chamber to the reservoir; and

e. a level-control switch for actuating the drive motor to operate the pump in one relative position of the cylinder and the piston, i.e. in a position in which the supported load (such as a motor vehicle) is at a relatively low level, and for actuating the electromagnet to open the regulating valve in an alternate position of the piston and the cylinder in which the load level is high.

This arrangement provides a suspension unit of compact construction which requires no hydraulic or pneumatic conduits mounted on the vehicle, and which nevertheless permits the desired level regulation to be achieved even when the vehicle is stationary.

Preferably, the energizing circuit of the pump-driving motor is connected to an external source of energy, and the levelcontrol switch is connected into the part of the circuit positioned within the suspension unit. Such a suspension unit can therefore be installed in a vehicle or used as a replacement for a normal shock absorber and acts as an automatic support member. The only fitting necessary is the connection of the power-supply leads to an electric current source aboard the vehicle by means of a simple plug connection, no additional supply pipes being required. Owing to the insertion of the level-control switch in the pan of the energizing circuit located within the suspension unit itself, even a vehicle with unbalanced load always travels in a level state since each suspension unit individually sets itself to the correct level.

The regulating valve is preferably electromagnetically actuated and is controlled by the same level-control switch which controls the motor. In this way both upward and downward adjustment to the desired level is achieved by purely electrical control in dependence on the switch reacting to the existing level of the vehicle.

In order to avoid repeated short-period switching of the motor driving the pump, and also of the regulating valve, caused by relative movements between the working cylinder and the piston rod arising during movement of the vehicle, a preferred feature of the invention comprises the insertion of time-lag relays into the electric circuit leads between the levelcontrol switch on the one hand and the motor and/or the regulating valve on the other hand.

Instead of providing such electrically operating delay means, I may subject the level-control switch itself to a correspondingly delayed action, as by operating its contacts through a dashpot assembly mechanically linked with the piston and the cylinder. This has the result that the switch is not responsive to the comparatively short-period oscillations between the working cylinder and the piston rod and only reacts to a definite displacement from the desired level.

The level-control switch is advantageously supported by the unit itself, either in or on the casing thereof. In the first instance the switch contacts may be disposed within the hollow rod of the working piston for operation, directly or indirectly, by a rigid element rising axially from an end of the cylinder; the aforedescribed dashpot assembly, if used to operate these contacts, may also be located inside the hollow piston rod, with replacement of the rigid element by a resilient linkage.

The construction and arrangement of the level-control switch and of the motor driving the pump may take many forms, and certain preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawing, in which:

FIG. 1 is an axial longitudinal section through a first embodiment of my improved suspension unit in accordance with the present invention;

FIG. 2 shows a hollow piston rod with a level-control switch and associated control means mounted therein;

FIG. 3 is a sectional view of part of a suspension unit with an electromagnetically operated regulating valve mounted in the base plug of the working cylinder;

FIG. 4 is a view, partly cut away, of part of a suspension unit with a pump and pump-driving motor mounted on the side of the outer cylindrical casing of the unit;

FIG. 5 is a sectional view through part of a further embodiment of my invention in which the level control is effected by sliding engagement of the working piston with the wall of the working cylinder;

FIG. 6 is a sectional view through part of a further embodiment with a level-control switch mounted in a hollow piston rod and actuated by a control piston displaceably mounted in that piston rod; and

FIG. 7 is an axial longitudinal section through another embodiment in accordance with the invention, in which the levelcontrol switch mounted in the hollow piston rod is actuated by a damping-control piston having a delayed-action characteristic.

The hydropneumatic suspension unit shown in FIG. 1 basically comprises an outer cylindrical casing 1, an inner cylinder 3 positioned concentrically inside the outer cylinder and defining a cylindrical working chamber 2, a base plug 4 secured to the lower end of the inner cylinder 3, a closure cap 5 at the upper end of the inner cylinder 3, and a piston rod 6 which extends through the closure cap 5 in sealing relationship therewith and which carries a piston head 7 at its inner end within the working chamber 2. The annular space between the inner cylinder 3 and the outer casing 1 is divided by an outer flange portion of plug 4 into upper and lower compartments or chambers by an annular fixedly mounted transverse partition 8. Between the transverse dividing wall 8 and the base plug 4, which extends radially outwardly of the inner cylinder 3, is a reservoir chamber 9 which is partially filled with working liquid. Above the dividing wall 8 is an annular chamber 10 which is in communication 'with the working chamber 2 by way of passages 11 and which, just like the working chamber 2, is completely filled with liquid. The working liquid in the working chamber 2 and in the upper annular chamber 10 is subjected to pressure exerted by a gas held within a chamber 12 which also lies above the dividing wall 8 but which is separated from the upper annular chamber 10 by a tubular membrane 13. The piston head 7 is formed as a dashpot piston and for this purpose is provided with two throttling passages 14 which are respectively effective in opposite directions and which are normally held closed or controlled by resilient valve plates 15.

The base plug 4 is provided with a number of radial passageways 16 in each of which a pumping plunger 17 is displaceably mounted. Each of these plungers 17 has a head 17' at its radially inner end which is biased by a spring 18 into permanent contact with the end of a suitably shaped shaft 20, provided for example with cam surfaces 19, of an electric motor 21 mounted beneath the base plug 4. The radially outer ends of the radial passageways 16 form respective small pump chambers and are each connected to receive liquid from the reservoir chamber 9 by way of an inlet passage 23 provided with a check valve 22, each passageway 16 being further connected to pump liquid to the cylindrical working chamber 2 by way of an outlet passage 25 provided with a check valve 24. In FIG. 1 in the right-hand pump chamber only the inlet pipe 23 is shown and in the left-hand pump chamber only the outlet pipe 25 is shown, but this is for clarity in the drawing only and in practice each pump chamber is provided with both such passages 23 and 25.

Moreover, although not shown in FIG. I, a regulating valve is provided in the base plug 4, and is shown in detail in FIG. 3 As illustrated there, the regulating valve comprises a passageway 26 connecting the cylindrical working chamber 2 with the reservoir chamber 9, and a valve cone 27 connected to a valve rod 27 which cooperates with the tapered peripheral surface of the armature 28 of an electromagnet 29. In the position of the armature 28 illustrated in FIG. 3 the valve cone 27 is urged against its seating surface by a leaf spring 27" secured to the upper surface of the base plug 4 and engaging the flat top of the cone.

The level-dependent control of the pump-driving motor 21 and also of the regulating valve 26 to 29 is carried out by means of a level-control switch mounted in the suspension unit and time-lag relays connected into the power-supply leads for the switch. As shown in the upper right-hand half of FIG. I, the level-control switch comprises three switch contacts 30, 31 and 32 positioned one above the other. The switch contact 30 is secured to a mounting plate 33 carrying a folding bellows 34, the mounting plate 33 resting on the free outer end 6 of the piston rod 6. The switch contacts 31 and 32 are held in the folds of the bellows 34 in a suitable manner. The central switch contact 31 has a lead permanently connected to the positive terminal of a power source mounted on the vehicle, the contact 30 is connected to a control lead 101 for the electromagnet 29 of the regulating valve 27, and the contact 32 is connected to a control lead, 102 for the pump-driving motor 21. Time-lag relays 101', 102' are inserted into the respective control leads 101 and 102.

In the position of the piston rod 6 shown in unbroken outline in FIG. 1 and which corresponds to the desired level position, the circuit connection between the switch contacts 31 and 32 is broken so that the pump-driving motor 21 is not energized. On the other hand, the control lead to the electromagnetic valve 27-29 through the switch contacts 30 and 31 is complete when the switch contacts are in this position, with the result that the regulating valve 27 is held closed. If the suspension unit is compressed to an extent corresponding to the position of the piston rod 6 and of the mounting plate 33 shown in broken outline in FIG. 1, then the switch contacts 31 and 32 close and complete the control circuit for the pumpdriving motor 21, with the result that the motor 21 becomes energized. The motor 21 reciprocates the pumping plungers 17 to cause working liquid to be pumped from the reservoir chamber 9 into the cylinder chamber 2, with a corresponding rise in pressure in the working chamber which forces the piston rod 6 back up to the desired-level position, whereupon opening of the switch contacts 31 and 32 causes the motor 21 to be turned off. If the piston rod 6 moves upward from the desired-level position, for example after unloading of the vehicle, the associated mounting plate 33 moves to the position shown in the upper right-hand corner of FIG. 1 in chaindotted lines, and the separation of the contacts 31 and 30 which occurs in this situation then causes the control circuit for the regulating valve 27-29 to be broken, so that the regulating valve 27 opens and a suitable quantity of working liquid can pass from the cylinder chamber 2 back to the reservoir chamber 9. The time-lag relays connected in the control leads ensure that the pump-driving motor 21 and also the regulating valve 27-29 are actuated only if a level regulation is really necessary, as for example during movement of the vehicle or as the result of loading or unloading thereof.

As is shown in the upper left-hand half of FIG. 1, the various switch contacts 30, 31, 32 of the level-control switch may alternatively be replaced by a suitably constructed microswitch 35 which is mounted between two adjacent coils of a flat spring 36 positioned between the mounting plate 33, the piston rod 6 and the closure cap 5 for the working cylinder.

In the embodiment shown in FIG. 2 the level-control switch comprises a microswitch 35 mounted in a hollow piston rod 6 and having a plunger 35' which can be moved to three different switch positions and is actuated by control means mounted in the piston rod and fixed to the base plug 4 at the bottom of the working cylinder 3. For this purpose, a control rod 37 is fixed to the base plug 4 of the working cylinder 3 and extends through a bore 39 in the piston rod where it passes through a seal 38. A plate 40 is mounted on the inner end of the control rod 37 and supports two concentrically arranged outer and inner springs 41 and 42 respectively. The outer spring 41 is of lesser strength than the inner spring 42, the latter being also somewhat shorter axially than the spring 41. A control piston 43 is provided Within the hollow piston rod 6 between the two springs 41, 42 and the microswitch plunger 35', piston 43 being held or guided for limited axial displacement by a mating fixed stop 44. In the position of the control rod 37 shown in FIG. 2, which corresponds to the desiredlevel position, the microswitch plunger 35 is held in its central operating position by the spring 41 acting on the plunger through the control piston 43. In this position the spring 41 compensates for the effect of the biasing spring (not shown) within the microswitch 35. If the vehicle is then loaded so that the control rod 37 is forced correspondingly deeper into the piston rod 6, then the inner stronger spring 42 also comes into contact with the control piston 43 and the microswitch plunger 35' is compressively displaced since the combined force exerted by the two springs 41 and 42 overcomes the force of the microswitch biasing spring. This causes the pumpdriving motor 21 to be energized. If, on the other hand, the vehicle is unloaded from the steady-state condition, the control rod 37 moves outwardly through a corresponding distance from the piston rod 6, with the result that the outer spring 41 becomes relaxed to such an extent that the microswitch plunger 35' under the influence of its internal biasing spring now moves into its position of greatest projection from the microswitch housing, thereby causing the regulating valve 27-29 to be opened.

In the arrangement shown in FIG. 5 the construction of the level, control switch is such that the working piston 7 and the piston rod 6 are formed of an electrically conductive material, while the tubular member defining the working chamber 2 is divided into three zones 45, 46 and 47 which are axially positioned one above the other and are electrically insulated from one another. The zone 45 adjacent the outer end of the piston rod 6 is connected to a supply lead 45' for the electromagnetic regulating valve 27-29 and the zone 47 adjacent the base of the working chamber 2 is connected to a supply lead 47 for the pump-driving motor 21. Rings 48 and 49 of a plastic material serve to provide the insulation between the individual zones and are held fixedly in position by an outer sleeve 50. If the piston rod 6 is forced inward and the piston 7 moves down to the region of the zone 47, then the pump-driving motor 21 is switched into circuit through a time-lag relay until the unit regains the level where the piston 7 has returned to the region of the current-free zone 46. Regulation takes also place in the event of an excessive extension of the piston rod 6, in which case the piston 7 will have moved into the zone 45 and the regulating valve 27-29 will have been opened by a further time-lag relay. The piston rod 6 is electrically insulated from the closure cap 5 by a guide 51 of plastic material seated in the closure cap.

FIG. 6 shows a level-control mechanism in which a control rod 52 secured at one end to the base plug 4 of the working cylinder 3 is arranged to project into the hollow interior 6' of the piston rod 6 and extends into the piston rod through a seal 53. The control rod 52, according to its depth of penetration into the piston rod 6, displaces a control piston 55 which is sealingly guided in the piston rod and is biased by a return spring 54, this displacement of the control piston 55 being effected hydraulically by the compression and expansion of the liquid within the piston rod. The control piston 55 is connected to a switching plunger 56 of a microswitch 57. At its upper end the switching plunger 56 carries a plate 56' which, as indicated, is connected to the vehicle-mounted power source. A first switch contact 58 is connected for a control lead to the pump-driving motor 21 and a second switch contact 59 is connected to a control lead for the electromagnet 29 of the regulating valve. In the event of excessive compressive movement of the piston rod 6, the control piston 55 is moved correspondingly upwards relative to the piston rod by the liquid compressed in the chamber 6' by the control rod 52, so that the switch plate 56 comes into contact with the switch contact 58 and the pump-driving motor 21 is switched on. As the desired level is reached, the switch plate 56 returns to its center position between the two contacts 58 and 59. If on the other hand the piston rod 6 and consequently also the control rod 52 are moved too far up, for example during unloading, then the control piston 55 is correspondingly displaced downwards until the engagement of the plate 56' with the switch contact 59 opens the regulating valve 27-29.

As is shown in FIG. 4, the electric pump-driving motor 21 may be mounted on the side of the outer cylindrical casing ll of the suspension unit and still enable apumping action similar to that described in connection with FIG. 1. Since there is always space at one side of the suspension unit, this large mass can be secured to the chassis of the vehicle. The lateral mounting of the motor 21 and of the pump makes it possible to construct a suspension unit of short overall length since its axial dimensions are then only dependent on the length of the working cylinder 2. The connection of the pump inlet passages 23 with the reservoir chamber 9 and of the pump outlet passages 25 with the annular chamber 10, which is here connected to the working chamber 2 by a port 60, is effected by respective annular channels 61, 62 and corresponding passages 61', 62' provided in the outer cylindrical casing 1. The reciprocating plungers l7 driven by the cam 20 lie here parallel to the axis of cylinder 3.

The suspension unit shown in FIG. 7 is similar to that shown in FIG. I and corresponding parts in the two Figures are therefore indicated by the same reference numerals. However, in this further embodiment the means for carrying out level regulation is of basically different form and manner of operation and eliminates the need for time-lag relays connected in the electrical control leads. In this modifiied construction an ancillary dashpot piston 63 is displaceably mounted in the interior 6 of the piston rod which is in permanent communication with the working chamber 2. The dashpot piston 63 is, on the one hand, coupled to the switching plunger 35 of the microswitch 35 by way of a control rod 64 and, on the other hand, connected to the base plug 4 by a force-transmitting coupling comprising a compression spring 65 and a tension spring 66 positioned one above the other. A guide tube 67 secured to the base plug 4 projects into the inner chamber 6' of the piston rod 6 and is encircled by the whole of the tension spring 66 and by part of the compression spring 65. The two springs 65 and 66 are connected to each other by means of a common guide sleeve 68 which is mounted for sliding movement on the guide tube 67. If the vehicle is displaced from the desired-level position, then either the force of the compression spring 65 or the force of the tension spring 66 acts on the ancillary piston 63 which in turn is then correspondingly but more slowly displaced in accordance with the damping force of the dashpot assembly 6, 63 and thereby displaces the switching plunger 35' whose contact plate 35 engages either the switch contact 35" in the pump-motor circuit or the switch contact 35" in the regulating-valve circuit. The desired central or level position is determined by the force balance between the tension spring 66 and the compression spring 65 and by the inactive position of the microswitch 35. There is permanent pressure equalization between the chambers 6', 6" and 2 which are filled with working liquid under any loading of the suspension unit. In the level position illustrated in the drawing the tension spring 66 normally rests on blocks. Upon too great an inward movement of the piston rod 6 the dashpot piston 63 moves slowly upwards because of the increased compression of spring 65 so as to turn on the pumpdriving motor 21 by closure of the switch contacts 35" and 35". On the other hand, starting from the level position shown in the drawing, in the event of too great an outward movement of the piston rod 6 the dashpot piston 63 moves downwards by reason of the then stressed tension spring 66 acting through the guide sleeve 68 and the compression spring 65 in tandem therewith with the result that the regulating valve 27-29 is opened by the closure of the switch contacts 35 and 35".

It will be realized that within the scope of the present invention many modifications are possible particularly in regard to the level-control mechanism and the construction of the pump-driving motor. For example, the pump-driving motor may take the form of an electrical vibrator which directly drives the pumping plungers.

What I claim is:

l. A self-regulating hydraulic suspension unit comprising:

a casing forming a pressurized reservoir for a working liquid;

an upright cylinder in said housing;

a working piston in said cylinder defining therein a working chamber and having an outwardly projecting piston rod for elevating a load in response to hydraulic pressure from said chamber;

conduit means in said housing forming a first passage and a second passage for said liquid between said reservoir and said chamber;

normally inoperative pump means in said first passage;

electric drive means supported by said casing for operating said pump means to feed liquid from said reservoir to said chamber;

normally closed valve means in said second passage;

electric control means for opening said valve means to enable a return flow of liquid from said chamber to said reservoir;

and switch means individual to said unit responsive to the relative position of said cylinder and said piston for actuating said drive means to operate said pump means in a relatively low load position and for actuating said control means to open said valve means in a relatively high load position.

2. The combination defined in claim 1, further including delay means connected with said switch means for preventing actuation of said drive means and said control means in response to short term displacements of said piston and said cylinder from a normal relative position.

3. The combination defined in claim 2 wherein said drive means and said control means are provided with respective operating circuits controlled by said switch means, said delay means including time-lag relays in said operating circuits.

4. The combination defined in claim 2 wherein said delay means comprises a dashpot assembly mechanically linked with said piston and said cylinder, said switch means including contacts controlled by said dashpot assembly.

5. The combination defined in claim 4 wherein said piston rod is hollow and forms a liquid space communicating with said chamber, said dashpot assembly including an ancillary piston in said chamber and a resilient linkage between said ancillary piston and said cylinder.

6. The combination defined in claim 5 wherein said resilient linkage comprises a compression spring and a tension spring in tandem.

7. The combination defined in claim 6 wherein said cylinder is internally provided with an axially extending guide element entering said liquid space and serially traversing said springs.

8. The combination defined in claim 1 wherein said switch means is supported by said casing.

9. The combination defined in claim 8 wherein said switch means comprises a collapsible structure engaging said piston and said cylinder and contacts on said structure closable upon a predetermined change in size of said structure due to relative motion of said piston and said cylinder.

10. The combination defined in claim 9 wherein the projecting part of said piston rod is provided with a mounting plate confronting an end of said cylinder, said structure being inserted between said mounting late and said end.

l l. The combination define in claim 10 wherein said structure is a bellows having said contacts secured to respective folds thereof.

12. The combination defined in claim 10 wherein said structure is a flat coil spring and said switch means comprises a switch housing on a turn of said spring and a button on said switch housing bearing upon an adjoining turn of said spring, said contacts being disposed in said housing for opening and closing by said button.

13. The combination defined in claim 8 wherein said piston is conductive, said switch means comprising two axially separated, mutually insulated conductive portions of said cylinder alternatively engageable by said piston for completing respective circuits for the actuation of said drive means and said control means.

14. The combination defined in claim 8 wherein said piston rod is hollow, said switch means including contacts in said piston rod and plunger means in said piston rod connected with said cylinder for opening and closing said contacts.

15. The combination defined in claim 14 wherein said plunger means comprises a rigid element extending axially into said piston rod and a nest of springs of different strength interposed between said element and said contacts for establishing an inactive and two active positions for the latter, said contacts being biased to oppose displacement by said springs.

16. The combination defined in claim 14 wherein the interior of said piston rod forms a sealed fluid space filled with hydraulic liquid, said switch means including a hydraulically shiftable member in said fluid space cooperating with said contacts and an element rigid with said cylinder extending axially into said fluid space for exerting a variable hydraulic pressure upon said member.

17. The combination defined in claim 1 wherein said cylinder has an end closed by a plug with an outer flange portion partitioning said housing into a first compartment including said reservoir and a second compartment isolated therefrom, said pump means being disposed in said plug, said drive means being a motor disposed in said second compartment.

18. The combination defined in claim 1 wherein said pump means comprises a pair of reciprocating plungers alignedly supported on said casing, said drive means comprising a motor with a cam shaft between said plungers bearing upon confronting extremities of the latter.

19. The combination defined in claim 18 wherein plungers are substantially parallel to the axis of said cylinder and are mounted laterally on said casing together with said motor. 

1. A self-regulating hydraulic suspension unit comprising: a casing forming a pressurized reservoir for a working liquid; an upright cylinder in said housing; a working piston in said cylinder defining therein a working chamber and having an outwardly projecting piston rod for elevating a load in response to hydraulic pressure from said chamber; conduit means in said housing forming a first passage and a second passage for said liquid between said reservoir and said chamber; normally inoperative pump means in said first passage; electric drive means supported by said casing for operating said pump means to feed liquid from said reservoir to said chamber; normally closed valve means in said second passage; electric control means for opening said valve means to enable a return flow of liquid from said chamber to said reservoir; and switch means individual to said unit responsive to the relative position of said cylinder and said piston for actuating said drive means to operate said pump means in a relatively low load position and for actuating said control means to open said valve means in a relatively high load position.
 2. The combination defined in claim 1, further including delay means connected with said switch means for preventing actuation of said drive means and said control means in response to short term displacements of said piston and said cylinder from a normal relative position.
 3. The combination defined in claim 2 wherein said drive means and said control means are provided with respective operating circuits controlled by said switch means, said delay means including time-lag relays in said operating circuits.
 4. The combination defined in claim 2 wherein said delay means comprises a dashpot assembly mechanically linked with said piston and said cylinder, said switch means including contacts controlled by said dashpot assembly.
 5. The combination defined in claim 4 wherein said piston rod is hollow and forms a liquid space communicating with said chamber, said dashpot assembly including an ancillary piston in said chamber and a resilient linkage between said ancillary piston and said cylinder.
 6. The combination defined in claim 5 wherein said resilient linkage comprises a compression spring and a tension spring in tandem.
 7. The combination defined in claim 6 wherein said cylinder is internally provided with an axially extending guide element entering said liquid space and serially traversing said springs.
 8. The combination defined in claim 1 wherein said switch means is supported by said casing.
 9. The combination defined in claim 8 wherein said switch means comprises a collapsible structure engaging said piston and said cylinder and contacts on said structure closable upon a predetermined change in size of said structure due to relative motion of said piston and said cylinder.
 10. The combination defined in claim 9 wherein the projecting part of said piston rod is provided with a mounting plate confronting an end of said cylinder, said structure being inserted between said mounting plate and said end.
 11. The combination defined in claim 10 wherein said structure is a bellows having said contacts secured to respective folds thereof.
 12. The combination defined in claim 10 wherein said structure is a flat coil spring and said switch means comprises a switch housing on a turn of said spring and a button on said switch housing bearing upon an adjoining turn of said spring, said contacts being disposed in said housing for opening and closing by said button.
 13. The combination defined in claim 8 wherein said piston is conductive, said switch means comprising two axially separated, mutually insulated conductive portions of said cylinder alternatively engageable by said piston for completing respective circuits for the actuation of said drive means and said control means.
 14. The combination defined in claim 8 wherein said piston rod is hollow, said switch means including contacts in said piston rod and plunger means in said piston rod connected with said cylinder for opening and closing said contacts.
 15. The combination defined in claim 14 wherein said plunger means comprises a rigid element extending axially into said piston rod and a nest of springs of different strength interposed between said element and said contacts for establishing an inactive and two active positions for the latter, said contacts being biased to oppose displacement by said springs.
 16. The combination defined in claim 14 wherein the interior of said piston rod forms a sealed fluid space filled with hydraulic liquid, said switch means including a hydraulically shiftable member in said fluid space cooperating with said contacts and an element rigid with said cylinder extending axially into said fluid space foR exerting a variable hydraulic pressure upon said member.
 17. The combination defined in claim 1 wherein said cylinder has an end closed by a plug with an outer flange portion partitioning said housing into a first compartment including said reservoir and a second compartment isolated therefrom, said pump means being disposed in said plug, said drive means being a motor disposed in said second compartment.
 18. The combination defined in claim 1 wherein said pump means comprises a pair of reciprocating plungers alignedly supported on said casing, said drive means comprising a motor with a cam shaft between said plungers bearing upon confronting extremities of the latter.
 19. The combination defined in claim 18 wherein plungers are substantially parallel to the axis of said cylinder and are mounted laterally on said casing together with said motor. 